This invention relates to digital video signal recording in a standard play (SP) mode or in a long play (LP) mode, with the digital video signals being recorded with a higher compression ratio in the LP mode than in the SP mode so that a lesser number of tracks may be used to record a frame in the LP mode than in the SP mode. More particularly, this invention is concerned with the recording of subcode data in respective tracks for both the SP and LP modes.
Digital video tape recorders capable of recording digital video signals on a magnetic tape in a cassette, known as D-VCR's, utilize data compression techniques for reducing the amount of data that must be recorded to permit accurate reproduction and display of a video picture. Typically, data compression relies upon discrete cosine transformation (DCT) and variable length encoding. A video frame, after being digitized and compressed, is recorded in a number of tracks, with each track exhibiting the general format shown in FIG. 1. This format includes an ITI area located at the leading portion of each track, followed by an audio area, a video area and a subcode area in this order. The ITI area is used as a timing block to assure the proper positioning of the tape and locating of individual tracks during, for example, an after-recording operation which is used to edit the tape. The audio and video areas contain digital audio and digital video data which, typically, are compressed. Subcode data is recorded in the subcode area and contains information which is useful for quickly locating particular video pictures, frames and tracks during a high speed search. For example, time codes, track numbers, and the like are included in the subcode data that is recorded in the subcode area. Typically, one frame of a video picture that is represented as a video signal in the NTSC standard (also referred to herein as the 525/60 standard) is recorded in ten successive tracks and a video picture represented by the PAL standard (also referred to as the 625/50 standard) is recorded in twelve successive tracks.
FIG. 2A is a schematic representation of the subcode data recorded in the subcode area in each track (whether recorded in the NTSC or PAL standard); and it is seen that the subcode data is comprised of twelve sync blocks SB0-SB11. A single sync block is schematically illustrated in FIG. 2B and is seen to include twelve bytes. The structure of a sync block is the same for each of sync blocks SB0-SB11; but the data content of the respective sync blocks varies, as will be described.
The twelve bytes of a sync block include two bytes which form a synchronizing pattern SYNC followed by two bytes of identifying data ID0 and ID1. A parity byte IDP follows the ID byte ID1 and is used to correct errors that may be present in the identifying bytes. Five bytes of data are recorded after the parity byte IDP, these five bytes being recorded in a data structure known as a pack which is described further below. Two parity bytes follow the data pack, thereby constituting the twelve byte sync block. Those familiar with digital video recorders will appreciate that the sync block included in the subcode data is shorter (i.e. it includes a smaller number of bytes) than the sync blocks which are included in the audio and video areas.
The identifying bytes ID0 and ID1 that are included in the subcode data recorded in a given track are not the same for all of the sync blocks included in that subcode data. For example, FIG. 2C schematically illustrates the identifying bytes ID0 and ID1 for sync blocks SB0 and SB6; and FIG. 2D illustrates the identifying bytes ID0 and ID1 for the sync blocks SB1-SB5 and SB7-SB11. In the example illustrated, the identifying bytes of sync blocks SB0 and SB6 differ from the identifying bytes of the remaining sync blocks. Byte ID0 in all of the sync blocks includes a flag F/R which is used in a high speed search to detect the first track in a frame of digital video signals. This F/R flag also is helpful in detecting an address not only for that track but for other tracks; and for convenience, this flag is referred to herein as an address-detecting flag. However, this term is not intended to be used as a limitation or as a strict definition for this F/R flag but, rather, is used simply to identify this flag.
As seen in FIG. 2C, the ID0 byte in sync block SB0 (and also in sync block SB6) includes a 3-bit application identifier AP3 followed by a digital representation of an absolute track number. The absolute track number is represented by eight bits, four of which are included in the ID0 byte and the remaining four are included in the ID1 byte. The last four bits of the ID1 byte for sync block SB0 (and also for sync block SB6) represent a sync number.
The data of the ID0 byte in sync blocks SB1-SB5 and SB7-SB11 differ from the data of the ID0 byte for sync blocks SB0 and SB6 in that, as shown in FIG. 2D, the application identifying bits AP3 found in sync blocks SB0 and SB6 are replaced by three bits which represent an index identifier, a skip identifier and a photopicture identifier. The index identifier is used during an index search; the skip identifier is used to indicate when a particular frame is to be skipped during a search and playback mode, and the photopicture identifier is used to identify a frame which represents a still picture. The remaining bits of the ID0 byte and of the ID1 byte in sync blocks SB1-SB5 and SB7-SB11 represent the same data as the corresponding bits in the ID0 and ID1 bytes in sync blocks SB0 and SB6.
Some of the sync blocks included in the subcode data are designated as "main area" sync blocks and the remaining sync blocks in the subcode data are designated as "optional area" sync blocks. For example, sync blocks SB3, SB4, SB5, SB9, SB10 and SB11 are main area sync blocks; whereas sync blocks SB0, SB1, SB2, SB6, SB7 and SB8 are optional area sync blocks. This different designation is used in conjunction with the five bytes of data that are present in each sync block, as shown in FIG. 2B, and as referred to hereinabove as the data pack.
When data is recorded in accordance with the NTSC (or 525/60) standard, a video frame is recorded in ten tracks, with the type of subcode data recorded in the data packs of the first five tracks (or first half of the frame) being different from the type of subcode data that is recorded in the data packs of the second five tracks (or second half) of the video frame. FIG. 3 is a table which represents the type of subcode data that is recorded in the main area of the subcode data in the first half of the video frame and in the second half of the video frame. It is seen that in the first half of the video frame, the data pack recorded in sync blocks SB3, SB5, SB9 and SB11 of each subcode area represents TTC (or title time code) data, and this pack simply is referred to as the TTC pack. In sync blocks SB4 and SB10 of the first half of the video frame, the data pack may be a TTC pack or, if the digital video signal is recorded for business use (as opposed to personal use), the data pack may be a title binary group (TBG) pack. Typically, for personal use, that is, for the usual consumer use, TBG packs are not recorded. As yet another alternative, the data packs recorded in sync blocks SB4 and SB10 in the first half of a video frame may represent no useful information, and such data packs are referred to as "no info" (NOI) packs.
In those tracks which constitute the second half of the video frame, TTC packs are recorded in sync blocks SB3 and SB9. The data packs included in sync blocks SB4 and SB10 may represent the date (year-month-day) on which the video signal is recorded, known as the VRD pack or they may represent the data on which the audio data is recorded, known as the ARD pack. The user may select whether the VRD or the ARD pack is recorded. If neither pack is included in these sync blocks, the data pack therein is referred to as the NOI pack.
In sync blocks SB5 and SB11 of the subcode data recorded in those tracks which constitute the second half of the video frame, a data pack is recorded representing the time (hour-minute-second) at which the video signal is recorded, and this is known as the VRT pack. Alternatively, the data pack may represent the time at which the audio signal is recorded, and this data pack then is known as the ART pack. If time data is not included in sync blocks SB5 and SB11, the data pack therein is represented as the NOI pack.
The subcode data recorded in the optional areas, namely sync blocks SB0, SB1, SB2, SB6, SB7 and SB8, may be user-designated; and in proposals that have been made heretofore for D-VCR's, it has been recommended that if there is no user designation for the recording of subcode data in the optional areas, such optional areas should record the same subcode data as are recorded in the main areas of that track.
The foregoing description of FIG. 3 has assumed the recording of NTSC (525/60) data in ten tracks. The same subcode data configuration may be used for the recording of PAL (625/50) video data in twelve tracks. Of course, in the PAL (625/50) standard, each half of a video frame consists of six tracks. Thus, in both the NTSC (525/60) and PAL (625/50) standards, the subcode data which is recorded in the first half of the video frame is of a different type than the subcode data which is recorded in the second half of the video frame, as is evident from the subcode data recorded in sync blocks SB4, SB5, SB10 and SB11 in each half.
A data pack which constitutes the TTC, TBG, VRD, ARD, VRT, ART or NOI pack exhibits the data structure shown in FIG. 4. Byte PC0 is referred to as the header, which identifies the data pack (for example, the header identifies the data pack as the TTC, TBG, VRD, ART, VRT, ART or NOI pack) and bytes PC1-PC4 constitute the appropriate data, such as date, time, etc. It is appreciated that each sync block, such as shown in FIG. 2B, contains only one data pack.
The subcode data shown in FIGS. 2A-2D and 3 are recorded with the compressed digital video signal in the standard play (SP) mode. It has been proposed that digital video signals may be data-compressed with a higher degree of compression for recording in a long play (LP) mode, such that the ten tracks of data that are used to represent one NTSC frame may be reduced to five tracks of data (similarly, the twelve tracks of data that are used to represent a PAL frame may be reduced to six tracks of data). By reducing by one-half the number of tracks needed to record a frame of digital video data, twice the amount of data may be recorded; and as a result, the recording time for a given length of tape may be doubled. That is, twice the amount of data or twice the duration of a video program may be recorded in the LP mode than may be recorded in the SP mode.
When data is recorded in the SP mode, ten tracks in the NTSC system and twelve tracks in the PAL system constitute one unit. The recording of subcode data is based upon this unit; and as is shown in FIG. 3, the type of subcode data that is recorded in the first half of this unit differs from the type of subcode data that is recorded in the second half of this unit. Nevertheless, high speed searching as well as the compilation of title, time, date, duration, etc., all of which are useful for searching or for display to a user, are based upon the 10-track (or 12-track) unit. Conveniently, this 10-track (or 12-track) unit constitutes a single frame unit and searching, as well as compilation, thus may be based upon a single frame unit. However, when video data is recorded in the LP mode, the 10-track (or 12-track) unit now represents two frames; and searching as well as compilation now is dependent upon a 2-frame unit. If the subcode data recording mechanism and algorithms which are used to record the subcode data during the SP mode (as represented in FIG. 3) also are used to record the subcode data in the LP mode, it is appreciated that the type of subcode data that is recorded with one LP frame (such as an odd frame) differs from the type of subcode data that is recorded with the following (or even) LP frame. This is because, when using the same subcode data mechanism or algorithm, the type of subcode data that is recorded in the first five (or six) tracks differs from the type of subcode data that is recorded in the second five (or six) tracks. As a result, whereas video data that is recorded in the SP mode can be edited on a frame-by-frame basis, that is, on the basis of a single frame unit, video data that is recorded in the LP mode can be edited only on a 2-frame unit basis. This is particularly troublesome when a common video tape has one video program recorded thereon in the SP mode and another video program recorded in the LP mode.
Another difficulty that arises when the video data may be recorded in the SP or LP mode relates to the state of the F/R flag. In the SP mode, the F/R flag exhibits a "1" during those tracks which constitute the first half of the video frame and a "0" in those tracks which constitute the second half of the video frame. FIGS. 5A and 5B illustrate the F/R flag for the NTSC standard. It is seen, therefore, that the F/R flag changes over from "0" to "1" at the first track in each frame and changes over from a "1" to a "0" at the sixth track of the 10-track frame. Since the change-over of the F/R flag as well as its state can be used to locate a particular track, the F/R flag is referred to herein as an address-detecting flag and is used during a high speed search to position the tape at the beginning of a frame.
If the same subcode recording mechanism or algorithm is used during the LP mode, the F/R flag changes over between "0" and "1" every five tracks which, for the NTSC system, means that the F/R flag changes its state at the beginning of each frame, as shown in FIGS. 6A-6B. However, if the beginning of a frame is detected by sensing the change-over of the F/R flag from "0" to "1", this technique will permit the beginning of only every other frame, such as the beginning of every odd frame, to be sensed because the F/R flag changes over from "1" to "0" at the beginning of every even frame. Thus, by sensing "0" to "1" transitions, the beginning of the even frames will not be detected.
As a result of this discrepancy, it is necessary, during a search operation, to provide some indication of whether the video signal on the digital video tape had been recorded in the SP or LP mode. Depending upon the mode used for recording, different searching techniques are used to locate a desired video picture or to identify the beginning of a video frame. Optimally, the algorithm shown in FIG. 7 is carried out before a searching operation is initiated. As depicted by the flow chart shown in FIG. 7, before a searching operation is initiated, inquiry S101 should be made to determine whether the video data to be searched was recorded in the SP or LP mode. If it is determined that the video data was recorded in the SP mode, inquiry S102 is made to determine when the F/R flags undergoes a positive transition from "0" to "1". When this inquiry is answered in the affirmative, instruction S103 is carried out to indicate that the first track in a frame is in position to be played back. However, if inquiry S101 determines that the video data had been recorded in the LP mode, inquiry S104 is made to sense when the F/R flag undergoes either a positive or negative transition. When such a transition is sensed, instruction S105 is carried out to indicate that the first track in a frame has been reached and is in position to be played back.
While the algorithm shown in FIG. 7 appears to be readily implemented, this algorithm is dependent upon inquiry S101 which, in many instances, is far more complicated than would appear. It would seem that if the video data recorded on a video cassette is known to have been recorded in the SP or LP mode, it would be a relatively simple matter to implement and respond to inquiry S101. For example, a user may provide a simple indication, such as providing suitable visual indicia on the video cassette, to indicate whether the video data recorded thereon has been recorded in the SP or LP mode. In some instances, however, a single video cassette may include video data that is recorded in one portion thereof in the SP mode and in another portion thereof in the LP mode. The recording of video data in mixed modes does not permit simple visual indicia to identify the mode or modes in which the video data had been recorded. Consequently, it is not a simple matter to determine whether inquiry S102 or inquiry S104 of the algorithm shown in FIG. 7 should be made. As a result, it would be difficult to search the video tape at a high speed, such as 200 times normal speed, to position the first track of the proper frame at the appropriate location for playing back that frame. It will be appreciated that, depending upon whether inquiry S102 or S104 is implemented, the video tape may be positioned at the beginning of the second half of a SP frame or, alternatively, the video tape may be positioned only at the beginning of odd LP frames, thus making it difficult, if not impossible, to position the tape at the beginning of an even LP frame.
If, for example, it cannot be determined prior to initiating a high speed search whether the video data had been recorded in the SP or LP mode, a search for a positive "0" to "1" transition in the F/R flag may be satisfactory to position the first track of each frame in the SP mode, but, as shown in FIGS. 8A and 8B, this technique will position the first track in only frames F2, F4, F6, etc. for those frames that had been recorded in the LP mode. Alternatively, if a negative transition from "1" to "0" of the F/R flag is sensed, it is seen that the first track in each of frames F3, F5, F7, etc. will be properly positioned for the video data recorded in the LP mode, but track 5 (the first track in the second half of a SP frame) will be positioned to play back the video data recorded in the SP mode. Typically, the sensing of the middle of a frame of video data during a high speed search is undesired and often unnecessary.